CO2 concentration measurement in dry gas mixtures

ABSTRACT

Described herein is an apparatus and methods for characterizing a fluid composition including exposing electrolyte to one fluid mixture, collecting a signal from an electrode in contact with the electrolyte, and simultaneously exposing the electrolyte to a second fluid, collecting a signal from a second electrode in contact with the electrolyte exposed to the second fluid, and comparing the signal difference between the electrodes with the Nerst equation wherein the temperature of the electrolyte is above 488° C. Carbon dioxide, nitrogen, and/or oxygen may be present in the fluid and/or the second fluid.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.13/966,093, filed Aug. 13, 2013, entitled “CO₂ concentration measurementin dry gas mixtures”.

FIELD

Embodiments herein relate to methods for monitoring carbon dioxideconcentration in fluid streams using an electrochemical method. It isreadily adapted for laboratory, wellbore, carbon dioxide pipeline, andflue-gas applications.

BACKGROUND

Measurement of CO₂ concentration in pipeline streams is useful andnecessary for a variety of reasons. The heating value of the natural gasstream, or more appropriately the standard heat of combustion,deteriorates with increasing CO₂ concentration.

In many CO₂ injection projects, methane is present in a recycle stream.Composition of the gas stream is regularly monitored for both separatoroperation and termination of inefficient recycling of CO₂. Furthermore,in some applications, the phase behavior of the gas stream is alteredsufficiently to have a material effect on operational design.

In contrast, many gas wells produce CO₂ with methane and other gaseouslight hydrocarbons. In these applications, a downhole measurement ofperiodic gas samples or an in-line continuous measurement is valuable.Likewise, in monitoring applications for fields undergoing CO₂injection, a robust method capable of quantitative determination forevaluating migration of CO₂ is essential. In these applications, sensorsensitivity spanning the entire range of mole fractions is required.

The Japanese patent application publication number 2000275213A uses adual molten carbonate cell where a first carbonate is dissociated togenerate CO₂ and O₂ by applying a small current of about 10 mA. Anothercell is used to measure the junction potential across the second cell'selectrolyte in contact with the reference gas stream and the sampledgas. The interpretation of the measured potential relies on small levelsof CO₂ concentration, typical of what is found in the atmosphere. Themechanical assembly is not designed to withstand a pressure differentialbetween the reference and sampling compartments.

The Japanese patent application publication number 2004170230A proposesa CO₂ sensor using electrodes with a film coated with a thick conductiveceramic, NASICON™. This type of apparatus generally suffers from limitedperformance in the presence of water vapor due to the deterioration ofNASICON™ with moisture.

U.S. Patent Application Publication Number 20090095626A1 uses a sensorstructure containing lithium phosphate as electrolyte and a mixture oflithium and barium carbonates as electrode surface coating materials.The reference electrode is a lithium titanate-titanium dioxide mixture.It operates at 500° C. This application's focus is structuring acarbonate electrode layer that had reduced sensitivity to humidity.

These references are applicable when the systems are at a low pressure,and the systems are designed to operate for dilute CO₂ concentration inthe gas stream. Furthermore, the sensors are slow to respond, oftenrequiring minutes. Given the corrosive nature of the electrolyte,feeding two different gas streams and preventing electrolyte migrationbeyond its housing remains an issue.

SUMMARY

Embodiments of the invention relate to an apparatus and methods forcharacterizing a fluid composition including exposing electrolyte to onefluid with a composition, collecting a signal from an electrode incontact with the electrolyte, exposing the electrolyte to a secondfluid, collecting a signal from a second electrode in contact with theelectrolyte exposed to the second fluid, and comparing the difference insignals to one using the Nerst equation wherein the temperature of theelectrolyte is above 488° C. Carbon dioxide is present in the fluidand/or the second fluid. Embodiments of the invention relate to methodsand an apparatus for observing a gas composition including a housingcomprising alumina configured to contain electrolyte, inlet and outletports in the housing for a fluid with a known composition, second inletand second outlet ports in the housing for a second fluid wherein thehousing directs fluid flow between the inlet and outlet ports and thesecond fluid flow between the second inlet and second outlet ports, anelectrode in contact with the electrolyte in contact with the fluid, andan electrode in contact with the electrolyte in contact with the secondfluid.

FIGURES

FIG. 1 (Prior Art) is a schematic view of an electrolyte based system tomeasure the electrochemical potential.

FIG. 2 is an electrolyte based system including a narrow slit forelectrolytic communication is provided at the bottom of the partitionbetween the two compartments.

FIG. 3 is a schematic of a tubular arrangement of the electrolyte withgold electrodes.

FIG. 4 is a schematic of a tubular gold sandwich arrangement of theelectrolyte. For clarity, the outside confining alumina is deliberatelyshown to be translucent.

FIG. 5 is a schematic of a sensor in a cell cup.

FIG. 6 is a plot of a comparison of Nernst potential with theexperimental data.

FIG. 7 is a plot of a response for CO₂ sample concentration.

FIG. 8 is a plot of a response for a range of temperatures.

FIG. 9 is a schematic of a pressure balanced sampling system.

DETAILED DESCRIPTION

Rapidity, robustness, and balanced high pressure systems areprerequisites in downhole applications for measuring carbon dioxideconcentration. Herein, we have shown a sensor assembly that enables usto measure CO₂ concentrations is a mixed-gas stream. The sensorelectrolyte is an eutectic mixture of carbonate compounds. The assemblyis constructed out of nearly inert alumina and provides separatepathways for the sample and reference gases. Incorporating a diffusioncontrolling communication channel between the two electrolytecompartments provides electrode positioning flexibility. We provide adiscussion of the separation and communication geometry of the housing.Additionally, pressure balanced sampling and delivery are alsodiscussed.

Herein, the sensor measures Nernst electrochemical potential using abinary mixture of Li₂CO₃ and K₂CO₃ as the electrolyte. This carbonatemixture forms an eutectic with a lowest melting point of 488° C. (seee.g. J. R. Selman and H. Maru's article in Advances in Molten SaltChemistry, edited by G. Mamantov, J., Braunstein, C., B. Mamantov, 4,1981). Several electrolyte mixture phase diagrams are presented in thereference, but the relevant one is for the Li₂CO₃—K₂CO₃ mixture). Anelectrochemical potential develops across the molten electrolyteinterfaces when sandwiched by gas streams of two different CO₂concentrations. The magnitude of the potential is given by the Nernstequation. This equation arises by relating the change in the Gibbs freeenergy (G) from reactants to products to the electrical potential (V)difference between the electrode on the product side to the reactantsside of the reaction, i.e.,ΔG=−nFΔV,  (1)where F is the Faraday constant, and n is the stoichiometric coefficientof electrons in the reaction. At standard conditions of reactants andproducts a subscript 0 is used for G and V, thus defining standardpotential with respect to which an electrical potential may be computedfor arbitrary concentrations. If the reactant species are labeled R_(i)and the products P_(j), with their stoichiometric coefficients beingα_(i) and β_(j), the electrical potential is (See, A. J. Bard and L. R.Faulkner, Electrochemical Methods and Applications, Wiley, 1980),

$\begin{matrix}{{V = {V_{0} - {\frac{RT}{n\; F}{\ln\left( \frac{\prod\limits_{j}a_{P_{j}}^{\beta_{j}}}{\prod\limits_{i}a_{R_{i}}^{\alpha_{i}}} \right)}}}},} & (2)\end{matrix}$where the activity is labeled a. For ideal mixtures, we may replace theactivity with partial pressures.

Carbonate mixtures function as electrolyte at unusually hightemperatures. For the one considered here, the operational temperatureis close to 500° C. Given the corrosive nature of the electrolyte,junction potential measurement is not trivial. Furthermore, mixingbetween the two gas streams, one of which is the reference, will corruptthe electrical potential measurement. While the concept of the Nernstequation and the junction potential is known, a robust sensor to measurethis potential unambiguously is desirable.

We describe a compact sensor that allows us to measure junctionpotentials rapidly, with equilibration time amounting to seconds. Thesensor includes a reference gas stream whose composition is known. Insome embodiments, this reference gas is controlled and contacts thefirst surface of the electrolyte. A gas mixture whose composition is tobe determined, henceforth called sampled gas or second fluid, contacts asecond surface of the same electrolyte. A Gibbs free energy (G)relationship between the equilibrated concentrations translates to anelectrical potential because the interfaces attain chemical equilibriumquickly with the contacting gases. It is this potential that relates theratios of CO₂ concentrations between the gas streams contacting thesurfaces. Sampled gas concentration is obtained because the referencegas CO₂ fraction is known.

The reference composition may be generated in a number of ways: (i)metered gas from sample chambers carrying pure gases, (ii) premixed gasof known composition, and (iii) chemically or electrochemicallygenerated gas stream where stoichiometric ratios fix the composition.On-demand supply of gas streams is useful in remote locations.Alternatively, in some applications, the reference gas in contact withthe first surface may be encapsulated within the sensor, prior todownhole deployment. In some embodiments, we provide the equipment togenerate a reference gas containing CO₂ and O₂ in a particular ratio.

In any event, let us assume an ideal gas mixture. When a moltencarbonate mixture is in equilibrium with a stream of gas, in the absenceof hydrogen, hydrogen containing compounds, and carbon monoxide, theprimary reaction is that of

$\begin{matrix}{{CO}_{2} + {\frac{1}{2}O_{2}} + {2\;{\left. e^{-}\longrightarrow{CO}_{3}^{2 -} \right..}}} & (3)\end{matrix}$

Consider a molten carbonate electrolyte whose carbonate ions CO₃ ²⁻ isin equilibrated contact with two different CO₂ streams. Herein, thestreams are the sample and the reference. The Nernst equation presentedabove allows us to write the potential between the sample (s) and areference (r) sides of the electrolyte, equal to V_(s)−V_(r). Since V₀is the same for both, assuming ideal mixture, the developed potentialbecomes

$\begin{matrix}{{V_{s} - V_{r}} = {\frac{RT}{2F}{\ln\left\lbrack \frac{\left( {P^{3/2}y_{{CO}_{2}}y_{O_{2}}^{1/2}} \right)_{s}}{\left( {P^{3/2}y_{{CO}_{2}}y_{O_{2}}^{1/2}} \right)_{r}} \right\rbrack}}} & (4)\end{matrix}$

If the pressure is balanced on both the sample and the reference side,almost a prerequisite if one is required to keep the electrolyte inplace, the above equation reduces to

$\begin{matrix}{{V_{s} - V_{r}} = {\frac{RT}{2F}{\ln\left\lbrack \frac{\left( {y_{{CO}_{2}}y_{O_{2}}^{1/2}} \right)_{s}}{\left( {y_{{CO}_{2}}y_{O_{2}}^{1/2}} \right)_{r}} \right\rbrack}}} & (5)\end{matrix}$

We have conducted our experiments with a continuously metered flow q, atstandard conditions, of three gases: O₂, N₂, and CO₂. Let q_(t) denotethe total of the rates of the three gases i.e. q_(t)=q_(O) ₂ +q_(CO) ₂+q_(N) ₃ . Then for an ideal mixture,y _(i) q _(i) /q _(t)  (6)where i represents O₂ or CO₂.

In the schematic configuration of FIG. 1, CO₂ on both sides of theelectrolyte 101 reacts with the carbonate mixture in the molten state toform carbonate anion. The molten carbonate conducts carbonate anions,whose concentration in equilibrium with the respective gases, sample gas102 and reference gas 103 sets in an imbalance of electrical potential,in order for the electrochemical potential to be equal. It is importantthat the configuration is structured so the true junction potential ismeasured. Ideally, electrodes at the surface of the electrolyte wouldenable this. But with molten carbonates this is not usually possible,given its unknown wettability, expansion, and gas pressure differences.

While the equilibrium measurements are applicable to a surfacepotential, in reality, especially in a downhole configuration, and atthe operational temperature (500° C.), it is difficult to achieveprecise contact with the surface. There is no guarantee of theelectrolyte maintaining contact or the electrolyte creeping around theelectrode. In the former, the potential obtained is irrelevant and inthe latter the magnitude is reduced. Additionally, as the electrolytemelts, a pressure imbalance between the sample and the reference maylead to gas bubbling through the electrolyte resulting in unwantedmixing, sample contamination and therefore reducing the signalmagnitude.

We have conducted a number of experiments in a variety of mechanicalconfigurations. Most of these were unreliable. Some of them were unableto prevent mixing of gases through a molten electrolyte and others haddifficulty measuring interface potential difference. Based on ourlaboratory experience, we identified embodiments that provided a highdegree of reproducibility with a close match to the theoreticallyexpected values.

FIG. 2 is a drawing of the sensor device. The body 002 and the tubing(not shown) that provide gas inlet and outlet from the body are made ofalumina. Geometrically, the sensor consists of a lid or a cover 001 andthe main body 002. The main body 002 has two sections 003, separated byan alumina wall 008 which has a small opening at the bottom forelectrolytic continuity (not shown). Our experiments have been conductedwith a slit opening at the bottom of the wall; but one may also have awall with a small orifice opening at the bottom of the wall. The lid 001has small ports 005 for the electrode wires (not shown), made of gold.There are independent ports 007 for the sample and the reference. Thewires make contact with the electrolyte and are slightly immersed in theelectrolyte. The electrolyte chambers have a small gas headspace. Thegases on either side of the wall do not communicate directly, becausethe wall is bonded to the lid with a high temperature cement such asAremco Seal 613. The seal may be undone by raising the temperature past850° C. and so the sensor may be used, taken apart, cleaned, andrefilled. Porous alumina plugs 006 with an average pore size of about 50μm are provided at the gas inlet and outlet. While the electrochemicalpotentials are equal for carbonate ions, maximum gradient in chemicalpotential, or equivalently the electrical potential, is present at thechannel, slit, hole or other orifice (not shown). Therefore, thepotential difference measured will be nearly equal to what istheoretically expected as the junction potential difference.

Some embodiments may benefit imbedding the molten salt within a porousmatrix, such as porous ceramic. By doing so, the movement of theelectrolyte is reduced, but still maintains the connections of theelectrolyte channel. The electrolyte may be embedded in porous aluminaor lithium aluminate in some embodiments.

The device was heated in a furnance (not shown) with temperaturecontrol. The furnace is commercially available from Barnstead Thermolyneof Hamsted, N.H. Further, the introduction of the gases to the devicewas controlled by flow meters including Porter Massflow controller modelnumber MPC95, commercially available from Parker Hannifin of Hatfield,Pa.

The volume of the electrolyte, the housing for the electrolyte, and theoverall system are driven by practical contraints. The surface areas ofthe molten electrolyte in contact with the fluids are much larger thanthe cross-sectional area afforded by the slit. The gas flow rates arecontrolled to prevent entrainment of the molten carbonate and fouling ofthe exit tubing from the system. The volume of the housing of theelectrolyte is selected for optimum heat transfer and for heat control.A smaller volume should be selected for tailoring the heat transfer,but, also, the surface area of the carbonate in contact with the gasflows must be large enough for robust contact. A lab bench scale devicewill have a different volume than devices for wellbore, carbon dioxidepipeline, and flue gas applications.

Variations for the housing design include the following: (i) make thewall thick and have a slit open at the bottom; (ii) rather than a slithave an orifice; keep the orifice sufficiently large that there is noblockage caused by solid electrolyte, but not so large that the orificeis no longer diffusion controlling; (iii) make the wall wedge shapedwith the thickest at the bottom so that we may provide sufficientelectrolyte, while the orifice at the bottom will be longer in order tobe diffusion controlling; and (iv) include gold paste, gold paint, or acombination to replace all or part of the electrodes.

Other embodiments are shown in FIGS. 3, 4, and 5. FIG. 3 provides a morecomplicated sensor with an arrangement to have gold discs with tubulargas feed, the discs enveloping the electrolyte and pressed together bymale/female alumina cylinders, a picture of which is shown in FIG. 4.With this configuration, each experimental run responded similarly whilevarying the sample composition, but the numerical value of the data wasnot in agreement with the theoretical predictions. The configuration ofFIG. 5 showed a similar issue, and without a diffusion resistantpartition in the electrolyte that is needed between the electrodes, onedoes not expect the correct electrical potential. Pressure balancing isalso required.

Experimental Results

In all of the experiments, we flowed a mixture of O₂, N₂ and CO₂. Noother gases were considered. In all of these cases, knowing the flowingcompositions, a theoretical potential may be calculated from the abovementioned Nernst equation 5, and compared to the value of the stabilizedmeasurement. In FIG. 6 we show a comparison with the experimentallyobtained points from three different runs with a CO₂ mole fraction of0.022 to 0.667. All of the data following a change in composition on thesample side are shown, including the transient response data, after achange in CO₂ mole fraction in the sample stream, except thatapproximately the first ten seconds of data have been excluded. In about6 s, the equilibrium potential is established.

To emphasize time evolution, one of the responses is shown in FIG. 7.Rapid approach to equilibrium is evident when sequential changes incomposition are made. Stabilization is established in a few seconds. Theresponse is rapid, and by about 6 seconds the voltage obtained is withinthe noise of the system. The noise is caused by a number of factors:temperature fluctuations, flow rate variations, and noise in the voltagepickup. There is a step change in the CO₂ fraction of 2.22 percent to 44percent at about 910 seconds. It takes about 6 seconds to reach the newequalibrium potential.

In FIG. 8, we show the comparison between the theoretical and theexperimentally obtained values over a range of temperatures. Below 488°C., the electrolyte is a solid, and no interpretable signal is obtained.Here it is quite different from the Nernst potential. The trend ishowever similar to the equilibrium potential probably because of alow-level electrolytic conduction close to the melting point orvariability in temperature. Since the eutectic mixture has a meltingpoint of 488° C., no comparison with Nernst potential is possible.Clearly the data at 480° C. has a large deviation from the calculatedNernst potential. From 490° C. and above the measured values arepredicted by the ideal mixture assumption for calculating theequilibrium potential difference.

Thus far, we have tacitly assumed that the sampled gas pressure is thesame as that of the reference, necessary for the proper functioning ofthe carbonate sensor. Otherwise the electrolyte will be displaced to theside with the lower gas pressure, ultimately forcing its way past theporous plugs, and causing undesirable mixing of gas streams. Ulitmately,the migration of the electrolyte from the sensor chamber would lead tocatastrophic failure, once the electrode contact is lost or if the flowlines are plugged by the solidified electrolyte.

In some embodiments, P_(s) and P_(r) are roughly equal; this preventsbackflow of the molten carbonate to the gas line. A reservoir supply ofreference gas is kept to replenish the reference gas in the chamber whennecessary. Through a remotely operable valve, this supply chamber iskept isolated, except for occasional replenishment needs. One embodimentwill have a reference chamber with a mixture of gases in thestoichiometric ratio for dissociation of carbonate i.e., a molar ratioof CO₂:O₂ of 2:1. Then, throughout the monitoring cycle, the referencecomposition is unlikely to change due to dissociation of the carbonate.

The sample side is also initially filled with the same reference gas atthe same pressure P_(r). In one embodiment, the freshly sampled gas isallowed to accumulate in a separate chamber. By opening an isolationvalve, the sampled gas is allowed to replace the previously present gas,at a sufficiently slow rate. We may carry this out with a back pressurefrom a chamber kept at the same pressure as that of the reference sidethrough digital control of a piston. This prevents appreciable flow ofgas towards or away from the reference side and avoids pressureimbalance. Molten carbonate displacement will then be inconsequential.

Most practical applications will require sequential operation withmultiple samples. Prior to deployment within a tool, we suggest bleedingthe reference gas from the supply unit to the sensor reference side andthe sample side with a common back pressure. This may be carried out atthe well-head. Subsequently, we elevate the temperature to just abovethe melting point of the carbonate. Because of the common back pressure,the pressure in the two systems remain the same. During samplecomposition determination, the reference line (if necessary; this lineneed not be replenished) and the sample line are allowed to flow out ata slow flow-rate past the sensor into back-pressure chambers kept at thesame pressure. The same back pressure could be enabled by having bellowsconnecting the two pressure chambers or as mentioned before, or throughdigitally controlled pistons. An example of the former is provided inFIG. 9. In FIG. 9, gases flow into bellows for which there is a commonback-pressure (not shown).

Since the sensors are built for a balanced pressure between sensorinternals and the external, it will be preferable to have the entiresystem under pressure balance, i.e., the gas pressures to be the same asthe pressure outside the cell. The outside chamber could be filled withan inert gas, e.g., argon, and may also be pressure balanced with thesame back-pressure chamber or a chamber in pressure equilibrium with theback-pressure chamber. Back-pressure magnitude is arbitrary. Someembodiments may benefit when the pressure of the fluid and the secondfluid are within 0.5 psi of each other.

Some embodiments may use a controller to provide the comparison betweenthe signals from the electrolyte. The term “controller” should not beconstrued to limit the embodiments disclosed herein to any particulardevice type or system. The controller may include a computer system. Thecomputer system may also include a computer processor (e.g., amicroprocessor, microcontroller, digital signal processor, or generalpurpose computer) for executing any of the methods and processesdescribed above.

The computer system may further include a memory such as a semiconductormemory device (e.g., a RAM, ROM, PROM, EEPROM, or Flash-ProgrammableRAM), a magnetic memory device (e.g., a diskette or fixed disk), anoptical memory device (e.g., a CD-ROM), a PC card (e.g., PCMCIA card),or other memory device. This memory may be used to store data fromtransmitted signals, relative signals, and output signals.

Some of the methods and processes described above can be implemented ascomputer program logic for use with the computer processor. The computerprogram logic may be embodied in various forms, including a source codeform or a computer executable form. Source code may include a series ofcomputer program instructions in a variety of programming languages(e.g., an object code, an assembly language, or a high-level languagesuch as C, C++, or JAVA). Such computer instructions can be stored in anon-transitory computer readable medium (e.g., memory) and executed bythe computer processor. The computer instructions may be distributed inany form as a removable storage medium with accompanying printed orelectronic documentation (e.g., shrink wrapped software), preloaded witha computer system (e.g., on system ROM or fixed disk), or distributedfrom a server or electronic bulletin board over a communication system(e.g., the Internet or World Wide Web).

Alternatively or additionally, the controller may include discreteelectronic components coupled to a printed circuit board, integratedcircuitry (e.g., Application Specific Integrated Circuits (ASIC)),and/or programmable logic devices (e.g., a Field Programmable GateArrays (FPGA)). Any of the methods and processes described above can beimplemented using such logic devices.

We claim:
 1. A method for characterizing a fluid composition,comprising: exposing electrolyte to a first fluid with a composition,wherein the first fluid is associated with a first flow pathway;collecting a signal from first electrode in contact with theelectrolyte; exposing the electrolyte to a second fluid, wherein thesecond fluid is associated with a second flow pathway; pressurebalancing the first flow pathway and the second flow pathway; collectinga signal from a second electrode in contact with the electrolyte exposedto the second fluid; and comparing the signals using the Nernstequation, wherein the temperature of the electrolyte is between 488°C.-550° C.
 2. The method of claim 1, wherein the first fluid is exposedby controlling a first fluid pressure.
 3. The method of claim 1, whereinthe second fluid is exposed by controlling a second fluid pressure. 4.The method of claim 1, wherein the pressure of the first fluid and thesecond fluid are within 0.5 psi of each other.
 5. The method of claim 1,further comprising: adjusting the flow of the first fluid, the secondfluid, or both through a flowmeter.
 6. The method of claim 1, furthercomprising: prior to exposing electrolyte to the first fluid, premixinggases in a pressure chamber to control the composition.
 7. The method ofclaim 1, wherein the composition is controlled by generating pure gasesin a fixed ratio.
 8. The method of claim 1, wherein a composition of thesecond fluid is estimated by comparing the signals from the first andsecond electrodes wherein the electrolyte is in contact with the firstand second fluids.
 9. The method of claim 8, wherein the first fluidcomprises a gas selected from the group consisting of carbon dioxide,oxygen, nitrogen, and a combination thereof.
 10. The method of claim 8,wherein the second fluid comprises a gas selected from the groupconsisting of carbon dioxide, oxygen, nitrogen, and a combinationthereof.
 11. The method of claim 1, wherein the electrolyte in contactwith the first fluid and second fluid comprises carbonate.
 12. Themethod of claim 1, wherein the electrolyte in contact with the first andsecond fluids comprises lithium carbonate and potassium carbonate. 13.The method of claim 1, wherein the electrolyte is molten.
 14. The methodof claim 1, wherein the electrolyte is a solid at and below 480° C. 15.The method of claim 1, wherein the electrolyte includes a porous ceramicmaterial.